This invention relates to a system and method for measuring characteristics of corrugated board and, more particularly, a system and method for measuring glue-line characteristics on such corrugated board.
Corrugated board is formed by bonding, with a starch-based, aqueous glue, one or more layers of paper, called liner, to the outside of one or more fluted paper layers, called medium. The medium is first passed between two fluted, heated rolls (corrugating rolls) which deform it into the desired fluted shape. Glue, consisting primarily of starch and water, is then applied to the top of the medium's flutes, after which it is bonded to the top liner to form the singleface web. Glue is then applied to the bottom of the singleface web's flutes before it is finally bonded to the bottom liner to form the finished board. To achieve acceptable bond strength, and to minimize glue consumption, a uniform application of glue is preferred with respect to both time and cross-machine position (across the machine's width). To ensure acceptable bonding the temperature of the glue during and after its application to the flute tips should also fall within a preferred range.
Improving the uniformity of glue application, with respect to both time and cross-machine position allows the glue consumption rate to be reduced. Reducing glue consumption provides numerous benefits. A typical corrugating machine, producing about 3,100 tons of board per month, will consume about 125,000 lbs of glue per month, which represents a significant raw material expense. The applied glue is also about 80% water, about three quarters of which must be dried out of the board by steam-filled hot-plates after the board is formed. The energy used to generate this steam also represents a significant expense. In addition, board warp (which generates sub-standard board that must be scrapped) is caused by moisture gradients within the board, which are aggravated by excessive glue application.
By measuring the starch mass and/or its temperature following application, with respect to both time and cross-machine position, irregularities can be detected and corrected, providing more uniform glue application to increase board strength and reduce glue purchases, energy use, and board scrap.
To date, no on-line glue measurement techniques have been successfully commercialized. A recognized, potential technique would use the principle of infrared absorption to indirectly measure the starch mass after its application. The basic principles of this sensor technology are described in numerous documents, including U.S. Pat. No. 5,049,216. However, the primary infrared absorption band for starch overlaps that of cellulose, and the ratio of starch mass to the background paper mass is so small (about 2.5 lbs of starch per 125 lbs of board) that the glue component cannot be identified with adequate precision using conventional techniques (even a significant 5% variation in starch mass would typically represent a total fluctuation of only about 0.1% of a corrugated board's total mass).
To measure the glue's temperature close to the point of bonding, the temperature of the glue must be measured shortly after it has been applied to the medium. However, the difference between the temperature of the glue and that of the paper behind it cannot be detected with conventional non-contact temperature sensors such as infrared pyrometers, which are based on the principle of passive infrared emission (in a conventional application of an infrared pyrometer the sensor would measure the average temperature of the glue-covered surface).
The glue's starch is deposited only on the flute tips of the medium, forming glue-lines which span the width of the machine. To amplify a glue measurement signal (whether it is representative of the starch mass or glue temperature), and isolate it from a board background measurement signal, the measurement must be localized on the glue-lines. However, conventional sensor technologies are incapable of localizing the measurement along the glue-lines, which are typically only about 0.04 inches long in the machine-direction, and which will pass a given point on the machine at the rate of about 1,000 glue-lines per second, for a machine speed of 1,000 feet/minute.
It is therefore a principle object of the present invention to provide a system and method for measuring glue-line characteristics on corrugated board.
It is a further object of the present invention to measure glue-line characteristics on corrugated board during the manufacture of such corrugated board.
Still another object of the present invention is to provide a system and method for the on-line measurement of the uniformity of glue application during the manufacture of corrugated board.